20.02.08
Testing for reliability in rail
Electrical testing is not only essential to prove the safety and reliability of electrical systems, it is also a valuable tool for the prediction and prevention of costly failures. Megger, probably the UK’s best-known manufacturer of portable electrical instruments, has products to cover virtually all rail-related test requirements.
Megger has long been recognised as a leading innovator in electrical testing technology and as a dependable supplier of test equipment that combines convenience and performance with safety and reliability. The company’s enviable reputation in these areas is fully supported by its present product range, much of which is developed and manufactured at the company’s UK headquarters in Dover.
Many of the company’s products are of relevance to the rail industry and are already widely used in this sector. Of particular interest are the latest insulation testers and battery testers, which we will look at in this issue, as well as low resistance ohmmeters and time-domain reflectometers (TDRs), which we’ll cover in the next issue.
A key feature of Megger insulation testers such as the popular MIT400 1kV model is their extended measuring range. Instead of simply indicating “infinity” for insulation resistances above few tens of megohms, Megger testers give accurate readings up into the tens of gigohms range. This may not seem to matter, but an accurate measure of insulation resistance can provide valuable input for a preventative maintenance programme.
Suppose that a test on, say, a traction motor shows that its insulation resistance is 10 gigohms, but that another test a month later shows that this figure has fallen to 1 gigohm. While both of these values are probably satisfactory, such a dramatic fall indicates that a problem is possibly developing, and that further attention is needed. Remedial action can, therefore, be taken before a costly breakdown occurs.
For the higher voltage equipment used in the rail industry, Megger makes 5kV and 10kV insulation testers. A study by GM Electromotive Division in the USA has shown that testing at these voltages can reveal a significantly higher proportion of incipient faults than testing at 1kV.
Another area where regular testing can provide big benefits is in the maintenance of storage batteries, which are widely used in the rail industry. It is notoriously difficult to determine the condition of storage batteries, but without this knowledge it is impossible to be sure that they will perform properly when called upon to supply power.
The traditional method of battery testing is load cycling, where the battery is taken out of service and fully discharged to determine its true capacity. This method is accurate, as it replicates the conditions experienced by the battery under load. The Torkel battery load unit allows controlled discharging of the battery system and monitors the performance, ensuring a reliable supply. It is, however, not always convenient to disconnect the battery system for a load test. In these cases, impedance testing gives an excellent indication of the battery condition on line.
Megger’s BITE 3 battery tester not only measures impedance, but also float and ripple current, cell voltage and intercell connection resistance. Used with the company’s ProActive battery database management software, this data provides an accurate and dependable indication of battery health without the need to take the battery off line. Data obtained from regular testing provides a reliable indication of problem areas, and also allows the end-of-life point for the batteries to be accurately determined.
Insulation testing and battery condition monitoring have important roles to play in ensuring the highest levels of safety and equipment availability in the rail industry. Almost equally important, however, are low resistance testing and the rapid location of cable faults, subjects that will be covered in the next issue.
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